An energy subtraction method is an image shooting method for improving visualization of an interest region using a difference of X-ray absorption characteristics of a material depending on energy of the X-rays. More specifically, in the method, a subject is irradiated with X-ray beams having high energy and X-ray beams having low energy, the respective X-ray beams transmitted through the subject are captured as X-ray image data by an image detection unit, and energy subtraction image data is generated by subtracting both of the X-ray image data pieces.
Further, the interest region can be changed by changing a weight to the X-ray image data captured by the high-energy X-ray beams and a weight to the X-ray image data captured by the low-energy X-ray beams. By the change, energy subtraction image data in which tissue of bone portions is erased and soft tissue is extracted as an interest region, and in an opposite manner, energy subtraction image data in which the soft tissue is erased and the bone portion is extracted as the interest region can be obtained.
In the generation of the energy subtraction image data, if each of the energy of the high-energy X-ray beam and the low-energy X-ray beam is not appropriate, the interest region cannot be appropriately extracted. Thus, various methods have been discussed to solve the problem.
In Japanese Patent Application Laid-Open No. 2007-222311, at least in image shooting using the high energy X-ray beam, the energy is discretely changed at a plurality of steps, and X-ray image data is captured at each step. Then, subtraction processing is performed using at least two or more combinations of the X-ray image data captured by the high-energy X-ray beam and the X-ray image data captured by the low-energy X-ray beam, respectively. By this processing, as compared to a case in which only one shot of the X-ray image data captured by the high-energy X-ray beam and one shot of the X-ray image data captured by the low-energy X-ray beam are captured, the possibility of obtaining energy subtraction image data captured with appropriate energy is increased.
However, when the X-ray image data is captured by discretely changing the energy, if there is appropriate energy other than the discrete energy, it is not possible to obtain appropriate energy subtraction image data. Accordingly, in the case of the technique discussed in Japanese Patent Application Laid-Open No. 2007-222311, energy subtraction image data captured with appropriate energy may not be obtained in a single shooting.